For example, in Patent Literature 1, a failure detection apparatus of an air-fuel ratio control apparatus which includes air-fuel ratio sensors disposed at positions before and after a catalyst, respectively, is disclosed. In this apparatus, a difference between the outputs of the air-fuel ratio sensors disposed before and after the catalyst is used to determine a failure of the air-fuel ratio sensor that is arranged on the upstream side or a failure of a catalytic converter. Further, in this apparatus, the output of the air-fuel ratio sensor on the downstream side is corrected based on a standard output, and the output of the air-fuel ratio sensor on the upstream side is corrected using the air-fuel ratio sensor on the downstream side.